Higher plants synthesize fatty acids via a common metabolic pathway. In developing seeds, where fatty acids attached to triglycerides are stored as a source of energy for further germination, the fatty acid synthesis pathway is located in the plastids. The first step is the formation of acetyl-ACP (acyl carrier protein) from acetyl-CoA and ACP catalyzed by a short chain preferring condensing enzyme, β-ketoacyl-ACP synthase (K-S) III. Elongation of acetyl-ACP to 16- and 18-carbon fatty acids involves the cyclical action of the following sequence of reactions: condensation with a two-carbon unit from malonyl-ACP to form a longer β-ketoacyl-ACP (β-ketoacyl-ACP synthase), reduction of the keto-function to an alcohol (β-ketoacyl-ACP reductase), dehydration to form an enoyl-ACP (β-hydroxyacyl-ACP dehydrase), and finally reduction of the enoyl-ACP to form the elongated saturated acyl-ACP (enoyl-ACP reductase). β-ketoacyl-ACP synthase I (KAS I), is primarily responsible for elongation up to palmitoyl-ACP (C16:0), whereas β-ketoacyl-ACP synthase II (KAS II) is predominantly responsible for the final elongation to stearoyl-ACP (C18:0).
Genes encoding peptide components of β-ketoacyl-ACP synthases I and II have been cloned from a number of higher plant species, including castor (Ricinus communis) and Brassica species (U.S. Pat. No. 5,510,255). KAS I activity was associated with a single synthase protein factor having an approximate molecular weight of 50 kD (synthase factor B) and KAS II activity was associated with a combination of two synthase protein factors, the 50 kD synthase factor B and a 46 kd protein designated synthase factor A. Cloning and sequence of a plant gene encoding a KAS III protein has been reported by Tai and Jaworski (Plant Physiol. (1993) 103:1361-1367).
The end products of plant fatty acid synthetase activities are usually 16- and 18-carbon fatty acids. There are, however, several plant families that store large amounts of 8- to 14-carbon (medium-chain) fatty acids in their oilseeds. Recent studies with Umbellularia californica (California bay), a plant that produces seed oil rich in lauric acid (12:0), have demonstrated the existence of a medium-chain-specific isozyme of acyl-ACP thioesterase in the seed plastids. Subsequent purification of the 12:0-ACP thioesterase from Umbellularia californica led to the cloning of a thioesterase cDNA which was expressed in seeds of Arabidopsis and Brassica resulting in a substantial accumulation of lauric acid in the triglyceride pools of these transgenic seeds (U.S. Pat. No. 5,512,482). These results and subsequent studies with medium-chain thioesterases from other plant species have confirmed the chain-length-determining role of acyl-ACP thioesterases during de novo fatty acid biosynthesis (T. Voelker (1996) Genetic Engineering, Ed. J. K. Setlow, Vol. 18, pgs. 111-133).